This invention relates to improved batteries, and particularly to the improvement of the shelf life of dry cell and other batteries which self-discharge when stored in high temperature environments.
The Daniell cell one of the early practical sources of energy, was found to work best when operating continuously in a closed circuit. The Daniell cell includes a zinc electrode immersed in a solution of zinc ions and a copper electrode immersed in a solution of bivalent copper ions. The two solutions are separated usually with porous plate separators or by gravity. Under open circuit conditions copper ions migrate to and deposit on the zinc electrode. This in effect results in the formation of small short circuited cells which rapidly dissolve the zinc electrode. A low resistance connection between the electrodes has been used to maintain cells under short-circuited conditions to inhibit this copper ion attack of the zinc electrode which results in improved operation of the cell.
In general, when a battery is stored or is not being used, it tends to self-discharge which results in decreased capacity of the battery. In a Leclanche-type dry cell, it is thought that this may be due to polarization, deposition of insoluble salts at an interface close to the anode as a result of localized pH changes or high concentrations of zinc ion, or formation of hydrogen peroxide from dissolved oxygen which reacts with the manganese dioxide. At elevated temperatures, usually above 25.degree.C., the loss of capacity is increased. The higher the temperature, the more rapidly the battery loses its capacity. The self-discharge of the battery can be represented by the current which would have to be drawn externally to effect the same reduction in the capacity of the battery. Typical self-discharge characteristics at 50.degree.C. of size "D" dry cells are shown in Table I.
Table I ______________________________________ Average Effective Self-Discharge Current (SDC) of "D" Type Dry Cells at 50.degree.C. over Specified Periods ______________________________________ Period (days) Average SDC (milliamps) ______________________________________ 0-10 0.31 10-30 0.25 30-60 0.13 69-90 0.04 ______________________________________